Respiratory gas humidification is a method of heating and humidifying respiratory gas for a user or patient, particularly the heating and humidifying of respiratory gas to be inhaled by the user or patient requiring respiratory gas ventilation support, possibly over a prolonged period of time. Respiratory gas humidification is necessary to compensate for losses of heat and moisture when natural airway passages of the user or patient are bypassed or somehow otherwise restricted as necessitated by the equipment used in providing a secure airway path into and from the patient's lungs to conduct respiratory gas ventilation support. Respiratory gas humidification could reduce or avoid clinical complications, such as pulmonary infection, physical damage of lung tissue, and/or debilitation of lung function.
Generally, there are two known methods of respiratory gas humidification, namely, active respiratory gas humidification, and passive respiratory gas humidification.
Active respiratory gas humidification typically uses gas and water transporting conduits, and/or gas and water vapour transporting conduits, provided with a gas-heating means and connected to a water-vapour-generating means such as a humidifier control unit and water vessel containing the water for evaporation. Water vessels of the prior art, commonly called humidification chambers, and such inspiratory and expiratory conduits of the prior art, under certain conditions, are known to cause unintentional condensing of evaporated water; whereby the condensed water unintentionally accumulates in the inspiratory and/or expiratory conduits. Unintentional accumulation of condensed water poses a risk to patient respiration, due to potential occlusion of the gases pathway to and/or from the patient's lungs; and/or under-humidification of inhalatory breathing gases.
Passive respiratory gas humidification is conducted without, and independent from, any external energy source or external water supply. Typically, passive respiratory gas humidifiers act as heat and moisture Exchangers (HME) by withdrawing heat and moisture from expired gas from a user or patient, and supplying the heat and moisture to gas to be inspired by the user or patient.
The present invention relates to a humidified gas delivery system for use in active respiratory gas humidification, and ensures that a user or patient is supplied with conditioned respiratory gas. In active humidifying methods, moisture and heat is input to respiratory gas prior to inhalation by the user or patient. Performance data and safety-related requirements for active respiratory gas humidifiers are specified by the standard ISO 8185. According to that standard, the minimum water content of inspired respiratory gas is about 33 mg/L of gas flow and the maximum respiratory gas temperature for inhalation is about 42° C.
In order to deliver gas, for example humidified respiratory gas, to a user or patient, flexible piping, typically having a length of about 1 to about 2 meters and having a wall thickness in the range of about 0.4 to about 2 mm, is used to deliver gas from a gas supply means to the user or patient (inspiratory conduit), and to deliver gas from the user or patient (expiratory conduit) to an outlet (or back to the gas supply in the case of a mechanical ventilator or other gas flow generating or receiving means).
The moisture content of humidified respiratory gas is raised, which is achieved by artificially increasing the amount of water vapour in the respiratory gas. Optimally, the relative humidity of the respiratory gas is 100% when inhaled by the patient or user. Artificial increase of the amount of water vapour in the respiratory gas is achieved by the use of a humidifier—a device, which enriches the respiratory gas with water vapour, for example, whereby a flow of respiratory gas (from a gas supply) is guided in the presence of a reservoir of water. The reservoir of water is heated to facilitate water evaporation and the production of water vapour, which water vapour is transported from the surface of the water in the reservoir to the inspiratory conduit by the flow of respiratory gas.
However, the water vapour transported by a humidified gas of high, for example 100% or greater, relative humidity is more likely to condense and accumulate at the inner walls of the inspiratory conduit—a process known as “rain out”. Consequently, the inspiratory circuit must be regularly drained to remove the accumulated liquid to circumvent the accumulated liquid reaching the lungs of the user or the patient.
Similarly, the expiratory conduit, which delivers gas to an outlet, will deliver humidified gas of high, for example >100%, relative humidity from the lungs of the user or patient, and water vapour transported by the expired gas is likely to condense and accumulate in the expiratory circuit, requiring regular draining, particularly in the case of an expiratory conduit connected to a mechanical ventilator or other gas flow generating or receiving means.
The prior art teaches that the inspiratory and expiratory conduits incorporate heating elements to heat the respiratory gas and simultaneously heat the conduit walls to a suitable temperature, and to maintain a constant temperature across the entire length of both of the inspiratory and expiratory conduits, in order to reduce condensation.